1. Field Of The Invention
The present invention relates to a solid-state image sensing apparatus and more particularly to a solid-state image-sensing device equipped with a binary circuit for outputting an image-sense signal as a binary signal and a barcode reading device using this solid-state image-sensing apparatus to read-out, convert to binary signals and output barcodes.
2. Description Of Related Art
Barcodes are information concerning merchandise codes etc. which are displayed usually as combinations of lines of differing sizes on a medium etc. These are then read-out at a XXX or POS (point of sale) etc. and used in the analysis of, for example, the total sales and circulation of merchandise.
CCD image sensors are mainly used as the read-out means for optically reading-out these barcodes. With barcode reading devices using these CCD image sensors, the output of the CCD image sensor is sent to a binary circuit, the combination of lines of different sizes are taken out as binary information and this binary information is then detected. A method for obtaining the binary information where the level (voltage) of an image-sense signal is compared with a threshold voltage at a comparator is usually adopted in the case of this binary processing.
However, while the image-sense signal from the CCD image sensor is being binarized, it is difficult to carry out binary processing for a fixed threshold voltage because the barcode print surface reflections are not uniform due to the influence of depressions or rises in the surface on which the barcode is printed and the influence of light coming from outside.
Because of this, with the barcode reading apparatus of the related art, a circuit built outside of the CCD image sensor chip is used to invert the comparator in the case of a signal level change just before the image-sense signal goes above a fixed absolute value.
The following is a description based on FIG. 19 of a barcode reading device for the related art.
First, a CCD image sensor 100 comprises a sensor row 102 having a plurality of light-receiving portions 101, for converting incident light into signal charges corresponding to the amount of incident light and accumulating the charges, arrayed in a row, and a charge transfer register 104 comprised of CCDs for transferring signal charges read-out from each of the light-receiving portions 101 of the sensor row 102 by a read-out gate 103 in one direction.
A charge/voltage converter 105 made, for example, by floating diffusion region, for detecting the transferred signal charge and converting this signal charge to a voltage is provided at the final stage of the charge transfer register 104.
A buffer circuit 106 comprising, for example, a source follower circuit for performing current amplification on the output of the charge/voltage converter 105 is provided at the post stage of the charge/voltage converter 105. This buffer 106 is formed on the same substrate (chip) as the sensor row 102, the read-out gate 103 and the charge transfer register 104.
Then, a CCD output (image-sense signal) is derived from the output of the buffer 106 and sent to outside via an external terminal 107 to then be sent to a binary circuit 109 after being level-amplified by an amplifier 108. A floating binary circuit etc. employing diodes is used as the binary circuit 109.
This floating binary circuit comprises a comparator 110 and diodes 111 and 112 connected in reverse polarity across the input terminals of the comparator 110. This structure is such that the comparator 110 is inverted in the case of a signal level change just before the image-sense signal exceeds a fixed absolute value.
The input/output characteristics of the binary circuit 109 posses a hysterisis characteristic whereby, as shown in FIG. 20, in the case when the input voltage Vin is, for example, 0 V and the output voltage Vout is, for example, 5 V i.e. the power supply voltage, the output voltage Vout is inverted to 0 V when the input voltage Vin rises from about 0 V to 0.7 V i.e. the level of the diode voltage drop. Alternatively, when the input voltage Vin is, for example, 5 V and the output voltage Vout is 0 V, the output voltage Vout is inverted to 5 V when the input voltage Vin falls by about 0.7 V.
It therefore follows that the output of the binary circuit 109 is inverted when the current signal level of the image-sense signal changes by .+-.0.7 V or more with respect to the signal level of the previous image-sense signal and this inverted signal can then be taken out as binary information.
A characteristic of this floating binary circuit is that it is possible to carry out binarization using the relative degrees of light and shade of a barcode by capturing changes in a signal as binary.
However, when applications are made in barcode reading devices, in solid-state image-sense devices of the related art including the binary circuit, it is necessary to use an amplifier providing a high degree of amplification as the amplifier 108 for noise countermeasures and a binary circuit 109 employing diodes 111 and 112, which makes the circuit structure complicated and makes manufacture on the same chip as the image sensor 100 i.e. an on-chip structure, difficult to achieve.
Even if the circuit etc. can be made on-chip, giving consideration to the progress in the current situation of low voltage power supply operating voltages for the CCD image sensor 100, for example, 3 V, normal junction-type diodes have a dead-zone in their hysterisis characteristics corresponding to a drop in the voltage level of about 0.7 V. A voltage which is two to three volts larger than 0.7 V or greater is therefore necessary for signal amplification within the sensor and handling this kind of large signal amplification with a 3 V power supply CCD image sensor is extremely difficult.